Secondary Ion Mass Spectrometry (SIMS) is an extremely powerful technique for analyzing surfaces due to its excellent sensitivity, high dynamic range, very high mass resolution, and ability to differentiate between isotopes. The sample to be analyzed is bombarded with an ion beam (i.e. the primary ion beam) in order to extract ions from the sample (i.e. the secondary ion beam). The secondary ion beam is then separated according to each individual ion's mass to charge ratio by passing it through a mass spectrometer. Many types of spectrometers exist including magnetic sectors, time of flight and quadrupoles.
In a conventional magnetic sector mass spectrometer, the ions are extracted by applying a high strength electric field between the sample and an extraction electrode, typically by applying a high voltage to the sample. Ions are then transported to the magnetic sector and deviated by the magnetic field before hitting the detector. In double focusing designs, an additional electrostatic sector is included. The radius of the electrostatic and the radius of magnetic sectors are calculated to produce an achromatic mass dispersion.
In a floating design mass spectrometer, the ions are extracted by applying a low strength electric field, then post-accelerated through the flight tube of the spectrometer in direction of the detector by applying a floating electric potential, namely an electric potential sufficient to allow the ions to reach the detector. The advantages of such design are that the extraction of secondary ions at low voltage avoids the disturbance of the primary ion beam allowing for higher lateral resolution analysis.
International patent application published WO 2005/008719 A2 relates to a mass spectrometer that switches the polarity of the pole pieces by using a permanent magnet. In this specific disclosure, the energy which is given to the ion beam is given at the extraction system and the magnet assembly is used only as a way to deviate the ions. The design of the magnet assembly with the rotating permanent magnet located outside of a vacuum chamber has for purpose to eliminate the need for rotary seals on feedthroughs into the vacuum chamber. However, this specific configuration prevents the possibility of applying a (high) voltage onto the magnet and prevents thus the floating of the whole mass spectrometer.
Japanese patent application numbered JPS58-204684 relates to an electromagnet device for a mass spectrometer. The electromagnet device of this document is designed for sustaining the application of any arbitrary (high) voltage (between −3 kV and +3 kV) on the pole pieces of the magnet. This renders the adoption of a low voltage ion source possible. However in this document the pole pieces are individually mounted on separate isolating supports, rendering accurate alignment of the pole pieces and precise definition of the pole piece gap difficult.
One of the most common solutions for magnetic sector mass spectrometers is to surround the vacuum chamber in which the ions travel by an electromagnet. The disadvantage of this approach is that a larger gap between the pole pieces is necessary to arrange the vacuum chamber in between the pole pieces of the magnet. With an increased gap, the homogeneity of the magnetic field inside the magnet decreases due to the increase of fringing magnetic fields regions. In addition, larger coils become necessary to induce the electromagnetic field, or, for a same coil size, more current needs to be injected. This can cause heating issues. A second solution consists in placing the electromagnet assembly inside the vacuum chamber. This requires a much bigger vacuum chamber and has the additional disadvantage that a cooling water circuit needs to be placed inside the vacuum chamber, increasing the complexity and the cost of the system. Placing an electromagnet inside a vacuum chamber therefore causes technical problems due to heat dissipation.